1. Field of the Invention
Embodiments of the present invention relates to a display device, and more particularly to an organic light-emitting diode display device and a driving method thereof. Although embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for reducing a data line charging time and preventing a residual image problem to improve a display quality.
2. Description of the Related Art
Recently, various flat panel display devices have been developed to have less weight and a thinner profile as compared to a cathode ray tube. Such flat panel display devices include a liquid crystal display device (hereinafter, referred to as “LCD”), a field emission display device (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”) and an electro-luminescence display device, etc. However, each of these flat panel devices has advantages and drawbacks.
The PDP has light weight, thin profile and wide screen display capability because its structure and manufacturing process are simple, but yet it has low light-emission efficiency and large power consumption. An active matrix LCD employing a thin film transistor (hereinafter, referred to as “TFT”) as a switching device has the drawback in that it is difficult to manufacture as a wide screen display screen because a semiconductor manufacturing processes are used, but the active matrix display is in high demand since it is typically used for a display device of a notebook personal computer.
The EL device, a self-luminous device, is generally classified as either an inorganic EL device or an organic light-emitting diode device depending upon the material of a light-emitting layer. When compared with the LCD and the PDP, the EL device has the advantages of fast response speed, large light-emission efficiency, high brightness and a wide viewing angle.
FIG. 1 shows the structure of a related art organic light-emitting diode device. Referring to FIG. 1, the organic light-emitting diode device includes a transparent anode electrode, an organic compound layer and a cathode electrode formed sequentially on a glass substrate. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL and an electron injection layer. If a driving voltage is applied across the anode electrode and the cathode electrode, then a hole within the hole injection layer and an electron within the electron injection layer move toward the emission layer, respectively, to excite the emission layer, so that the emission layer emits visible rays. The visible rays generated from the emission layers of multiple pixels display a picture or a motion picture.
The organic light-emitting diode device can either be a passive matrix type or an active matrix type, which uses a TFT as switching element. In the passive matrix type device, the anode electrode crossing the cathode electrode are used to select a light-emitting cell in accordance with a current applied to the electrodes. In the active matrix type, an active element, such as a TFT, is turned on to select a light-emitting cell and maintains light-emission of the light-emitting cell by using a voltage maintained in a storage capacitor.
FIG. 2 is a schematic view of an organic light-emitting diode display device of a related art active matrix type. FIG. 3 is an equivalent circuit diagram of one pixel shown in FIG. 2. As shown in FIG. 2 and FIG. 3, the related art organic light-emitting diode display device has an organic light-emitting diode display panel 16 including pixels 22 respectively arranged at each intersection of gate lines GL and data lines DL, a gate driving circuit 18 for driving the gate lines GL, a data driving circuit 20 for driving the data lines DL and a timing controller 24 for controlling the gate driving circuit 18 and the data driving circuit 20.
The timing controller 24 controls the data driving circuit 20 and the gate driving circuit 18. To this end, the timing controller 24 supplies a variety of control signals to the data driving circuit 20 and the gate driving circuit 18. Further, the timing controller 24 re-aligns data to supply it to the data driving circuit 20.
The gate driving circuit 18 sequentially supplies a gate signal to the gate lines GL in response to a control signal from the timing controller 24. Herein, the gate signal is supplied in such a manner to have a width of one horizontal time 1H. The data driving circuit 20 supplies a video signal to the data lines DL by a control of the timing controller 24. In this case, the data driving circuit 20 supplies a video signal of one horizontal line to the data lines DL during one horizontal time 1H which the gate signal is supplied.
The pixels 22 emit light corresponding to a video signal, that is, a current signal supplied to the data lines DL, to thereby display a picture corresponding to the video signal. To this end, each of the pixels 22 includes an organic light-emitting diode device driving circuit 30 for driving an organic light-emitting diode device OLED in accordance with a driving signal supplied from each of the data lines DL and the gate lines GL. More specifically, the organic light-emitting diode device OLED is connected between the organic light-emitting diode device driving circuit 30 and a ground voltage source GND. The organic light-emitting diode device driving circuit 30 includes a first driving thin film transistor (hereinafter, referred to as “TFT”) T1 connected between a high-level potential driving voltage source VDD and the organic light-emitting diode device OLED, a first switching TFT T3 connected between the gate line GL and the data line DL, a second driving TFT T2 connected between the first switching TFT T3 and the high-level potential driving voltage source VDD to provide the first driving TFT T1 and a current mirror circuit, a second switching TFT T4 connected between the gate line GL and the second driving TFT T2, and a storage capacitor Cst connected between a node positioned between the first and second driving TFT T1 and T2 and the high-level potential driving voltage source VDD. Herein, the TFTs are a p-type Metal-Oxide Semiconductor Field Effect Transistor (hereinafter, referred to as “MOSFET”).
The gate element of the first driving TFT T1 is connected to the gate element of the second driving TFT T2, and a source element is connected to the high-level potential driving voltage source VDD. A drain element of the first driving TFT T1 is connected to the organic light-emitting diode device OLED. A source element of the second driving TFT T2 is connected to the high-level potential driving voltage source VDD, and a drain element is connected to a drain element of the first switching TFT T3 and a source element of the second switching TFT T4. A source element of the first switching TFT T3 is connected to the data line DL, and a gate element is connected to the gate line GL. A drain element of the second switching TFT T4 is connected to the gate elements of the first and second driving TFT T1 and T2 and the storage capacitor Cst. A gate element of the second switching TFT T4 is connected to the gate line GL. The first and second driving TFT T1 and T2 are connected in such a manner as to provide a current mirror. Accordingly, if the first and second driving TFT T1 and T2 have the same channel width, then the currents flowing in the first and second driving TFT T1 and T2 are equal.
An operation process of the organic light-emitting diode device driving circuit 30 will be described as follows. First, a gate signal is supplied from the gate line GL, which is a horizontal line. If the gate signal is supplied, then the first and second switching TFT T3 and T4 are turned-on. If the first and second switching TFT T3 and T4 are turned-on, then a video signal applied from the data line DL is supplied, via the first and second switching TFT T3 and T4, to the gate element of the first and second driving TFT T1 and T2. In this case, the first and second driving TFT T1 and T2 supplied with the video signal are turned-on.
The first driving TFT T1 adjusts a current flowing from the source element, that is, VDD of the first driving TFT T1 into the drain element in accordance with a video signal supplied to the gate element of the first driving TFT T1 to provide it to the organic light-emitting diode device OLED, so that the first driving TFT T1 controls light brightness of the organic light-emitting diode OLED corresponding to the video signal. Simultaneously, the second driving TFT T2 supplies, via the first switching TFT T3, a current Id supplied from the high-level potential driving voltage source VDD to the data line DL. Since the first and second driving TFT T1 and T2 form a current mirror circuit, then the same currents Id flow in the first and second driving TFT T1 and T2. Meanwhile, the storage capacitor Cst stores a voltage from the high-level potential driving voltage source VDD in such a manner as to correspond to the current Id flowing in the second driving TFT T2. Then, the storage capacitor Cst is turned-on by the first driving TFT T1 in response to the voltage stored in the storage capacitor Cst when the gate signal is off to be turned-off the first and second switching TFT T3 and T4, so that the storage capacitor Cst allows a current corresponding to the video signal to be supplied to the organic light-emitting diode device OLED.
A charging characteristics on the data line deteriorates due to the effect of a parasitic capacitance with the data line while driving at a low-level. When the related art organic light-emitting diode display device driven in accordance with a current drive method is driven at a low current level, then the problem of increased charging time occurs. To solve this problem, the related art organic light-emitting diode display device is implemented in such a manner as to be capable of scaling current by a proportional constant of T2/T1 on the condition that a function f1 for converting a data current Id into a data voltage Vp is linearly proportional to a function f2 for converting the data voltage Vp into the organic light-emitting diode device OLED current Ie1 in the organic light-emitting diode device driving circuit 30. But a proportional relationship between T2 and T1 is not always constantly maintained, and a difference between the pixels is generated by non-uniformities among the TFTs or a TFT deterioration. Accordingly, the related art organic light-emitting diode display device has a drawback of picture quality detioration. Because the related art organic light-emitting diode display device up-scales a current level in a constant ratio irregardless of a gray scale, there is also a problem in that a current for charging a data line is not enough in the case of a lower gray scale to be up-scaled in a relatively high ratio, and a bias stress of the driving TFT is increased for the case of a higher gray scale to be up-scaled in a relatively low ratio.